The biosynthesis of estrogens from circulating androgens, will be studied with emphasis on the mechanism and intermediates especially 19-hydroxy-C19-steroids. The assumption that the fraction of a hormone converted to its metabolites is constant during the measurement of its secretory rate by isotopic dilution in vivo will be tested for many hormones by injecting labeled hormone at a given time, and hormone labeled with different isotope at another time and then determining the isotopic ratio of metabolites isolated from the urine. Isotopic ratios different from the dose indicate a changing fractional conversion. The enterohepatic circulation of aldosterone and other C21-steroids will be studied by isolating and identifying labeled metabolites appearing in bile after injection of the labeled hormone. The role of intestinal flora in metabolism of biliary steroids will be studied in vitro. The metabolites of aldosterone will be synthesized for proof of structure and for use as carrier in isotope dilution experiment. Compounds thought likely to correspond to urinary metabolites of 19-hydroxyandrostenedione will also be synthesized.